For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well known Otto or Diesel cycles (the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Each stroke requires one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto or Diesel cycle in each cylinder of a conventional engine.
Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston is operable to reciprocate through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston is operable to reciprocate through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve operable to define a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi (herein the “Scuderi patent”), U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to David P. Branyon et al. (herein the “Branyon patent”), and U.S. Pat. No. 7,353,786 granted Apr. 8, 2008 to Scuderi et al. (herein the “Air-Hybrid patent”) each contain an extensive discussion of split-cycle and similar type engines. In addition the Scuderi, Branyon, and Air-Hybrid patents disclose details of prior versions of engines of which the present invention comprises a further development. The Scuderi, Branyon, and Air-Hybrid patents are each hereby incorporated by reference in their entirety.
Referring to FIG. 1, a prior art split-cycle engine of the type similar to those described in the Branyon and Scuderi patents is shown generally by numeral 50. The split-cycle engine 50 replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder 66 and one expansion cylinder 68. The four strokes of the Otto or Diesel cycle are “split” over the two cylinders 66 and 68 such that the compression cylinder contains the intake and compression strokes and the expansion cylinder 68 contains the expansion and exhaust strokes. The Otto or Diesel cycle is therefore completed in these two cylinders 66, 68 once per crankshaft 52 revolution (360 degrees CA).
During the intake stroke, intake air is drawn into the compression cylinder 66 through an inwardly opening (opening inward into the cylinder) poppet intake valve 82. During the compression stroke, the compression piston 72 pressurizes the air charge and drives the air charge through one or more crossover passages 78, which act as the intake passages for the expansion cylinder 68.
The volumetric compression ratio of the compression cylinder of a split-cycle engine is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric compression ratio of the expansion cylinder of a split-cycle engine is herein referred to as the “expansion ratio” of the split-cycle engine. Due to very high compression ratios (e.g., 40 to 1, 80 to 1, or greater), outwardly opening (opening outward away from the cylinder) poppet crossover compression (XovrC) valves 84 at the inlet of each of the one or more crossover passages 78 are used to control flow from the compression cylinder 66 into the one or more crossover passages 78. Due to very high expansion ratios (e.g., 40 to 1, 80 to 1, or greater), outwardly opening poppet crossover expansion (XovrE) valves at the outlet of each of the one or more crossover passages 78 control flow from the one or more crossover passages 78 into the expansion cylinder 68. The actuation rates and phasing of the XovrC and XovrE valves 84, 86 are timed to maintain pressure in the one or more crossover passages 78 at a high minimum pressure (typically 20 bar or higher at full load) during all four strokes of the Otto or Diesel cycle.
One or more fuel injectors 90 (one for each crossover passage 78) inject fuel into the pressurized air at the exit end of the one or more crossover passages 78 in correspondence with the XovrE valve(s) 86 opening, which occurs shortly before the expansion piston 74 reaches its top dead center position. The fuel-air charge fully enters the expansion cylinder 68 shortly after the expansion piston 74 reaches its top dead center position. As expansion piston 74 begins its descent from its top dead center position, and while the XovrE valve(s) 86 is/are still open, the spark plug 92 is fired to initiate combustion (typically between 10 to 20 degrees CA after top dead center of the expansion piston 30). The XovrE valve(s) 86 is/are then closed before the resulting combustion event can enter the one or more crossover passages 78. The combustion event drives the expansion piston 74 downward in a power stroke. Exhaust gases are pumped out of the expansion cylinder 68 through an inwardly opening poppet exhaust valve 88 during the exhaust stroke.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, compression ratio, etc.) of the compression and expansion cylinders are generally independent from one another. For example, the crank throws 56, 58 for the compression cylinder 66 and expansion cylinder 68 respectively may have different radii and may be phased apart from one another with top dead center (TDC) of the expansion piston 74 occurring prior to TDC of the compression piston 72. This independence, among other factors, enables the split-cycle engine to potentially achieve higher efficiency levels and greater torques than typical four stroke engines.
Considerable research has been devoted to air hybrid engines, which store energy for later use in the form of compressed air. The split-cycle engine 50 shown in FIG. 1 can be combined with an air tank and various control features to provide an air hybrid system.
FIG. 2 illustrates an exemplary prior art split-cycle air-hybrid engine. Referring to FIG. 2 in detail, a prior art split-cycle engine 50 is shown of the type described in FIG. 1. One or more of the one or more crossover passages 78 are connected to an air tank 94 via a control valve 93. Valve 93 is opened and closed at appropriate times to control the flow of air between the air tank 94 and the one or more crossover passages 78. Compressed air from the one or more crossover passages 78 is stored in the air tank at certain times such as, for example, when the vehicle is braking. The compressed air in the air tank 94 is fed back into the one or more crossover passages 78 at a later time in order to drive the crankshaft 54 in a pre-compressed air power (PAP) mode. The PAP mode can include a pre-compressed combustion-air power mode, wherein pre-compressed air and fuel are mixed and the fuel/air mixture is combusted to drive the power piston down during an expansion stroke. Further, the PAP mode can include various air motoring (AM) modes, wherein pre-compressed air is utilized to drive the power piston down during an expansion stroke without a corresponding combustion event occurring in the expansion cylinder. The Air-Hybrid patent describes details of the PAP modes of operation and other aspects of a split-cycle air hybrid engine similar to the one shown in FIG. 2.
The actuation mechanisms (not shown) for valves 82, 84, 86, 88 may be cam driven or camless. In general, a cam driven mechanism includes a camshaft mechanically linked to the crankshaft. A cam is mounted to the camshaft, and has a contoured surface that controls the profile of the valve lift (i.e. the valve lift from its valve seat, versus rotation of the crankshaft). A cam driven actuation mechanism is efficient and fast, but has limited flexibility.
Also in general, camless actuation systems for valves are known, and include systems that have one or more combinations of mechanical, hydraulic, pneumatic, and/or electrical components or the like. Camless systems allow for greater flexibility during operation, including, but not limited to, the ability to change the valve lift height and duration and/or deactivate the valve at selective times. Pneumatically actuated camless valves are generally advantageous for various reasons such as their compact packaging, low energy consumption requirements, and relatively low cost.
Dynamic actuation of the crossover valves 84, 86 of split-cycle engine 50 is very demanding. This is because the crossover valves 84 and 86 must achieve sufficient lift to fully transfer the fuel-air charge in a very short period of crankshaft rotation (generally in a range of about 30 to 60 degrees CA) relative to that of a conventional engine, which normally actuates the valves for a period of at least 180 degrees CA. This means that the crossover valves 84, 86 must actuate about four to six times faster than the valves of a conventional engine.
Valve springs (not shown) for the valves 82, 84, 86, 88 are used to keep the valves 82, 84, 86, 88 closed when they are not being actuated. Any suitable valve springs can be used for the intake valve 82 and the exhaust valve 88 such as mechanical springs or air springs. However, the crossover valves 84, 86 preferably use air springs because standard mechanical springs can have difficulty closing the crossover valves 84, 86 quickly enough to meet the aforementioned demanding crossover valve actuation requirements.
Pneumatic actuators, air springs, and other pneumatically powered components generally require a steady source of cool, dry, compressed air at a constant pressure that is free of particulates. These components generally need a steady source of such compressed air because, inter alia, compressed air tends to leak. Accordingly, there is a need in the art for providing such a compressed air source with an engine, more particularly with a split-cycle engine.